TDS refers to total dissolved solids in the water. Dissolved solids refer to any minerals, salts, metals, cations or anions dissolved in water. Total dissolved solids (TDS) comprise inorganic salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates) and some small amounts of organic matter that are dissolved in water. Removal of TDS from water is done through either an ion exchange process, such as a Water Softener or Ion Exchange Media, or reverse osmosis.
TDS is measured by TDS meters that measure the electrical charge in the water and is not an effective tool for measuring the effectiveness of a refrigerator filter. Refrigerator filters use a granular activated carbon block for the removal of chlorine and other organic molecules and do not affect TDS in the water. To get an accurate measure of how the filter is performing you would need to use a different type of testing for the filter, such as a chlorine test strip.